Brake control systems have been in widespread use for several years. Generally speaking, depressing a brake pedal allows pressure from a hydraulic supply to reach the brake. Brake pressure creates a force on brake rotors and stator which thru brake friction create a torque decelerating the wheel. The wheel starts to slip which creates a drag force on the axle slowing down the vehicle.
Antiskid brake control systems also have been in widespread use for many years. In the simplest sense, an antiskid brake control system compares the speed of a vehicle derived from a wheel speed sensor (and wheel radius) to the vehicle speed derived from a secondary or reference source. If the wheel is determined to be skidding an excessive amount, then brake pressure applied to the wheel is released and the wheel is allowed to spin back up to the appropriate speed.
Autobrake brake control systems for aircraft have also been in widespread use for many years. Essentially, autobrake functionality allows the pilot to arm the rejected take off (RTO) setting prior to takeoff or to select from several automatic deceleration levels for landing. After landing, pressure is automatically applied to the brakes after touchdown independent of the pilot's brake pedals. In multiwheel vehicles, the same pressure is usually applied to all the wheels. The system regulates brake pressure to compensate for the effects such as aircraft drag, thrust reversers, and spoilers to maintain the selected deceleration level. A typical autobrake system has at least three levels of deceleration: low, medium, and maximum. Depending on the selected level of deceleration, the plane will automatically decelerate after landing.
In a manner similar to the autobrake systems, deceleration through pilot-controlled braking can also be assisted or controlled. A pilot-controlled system might function to obtain a desired deceleration from pilot pedals, rather than an autobrake setting. The desired deceleration setting is then used to scale the desired deceleration to a value between zero and the maximum deceleration of which the vehicle is capable based on factors such as aircraft geometry and tire/runway friction.
There are, of course, major problems that immediately become apparent in any brake control system. One problem is that antiskid, autobrake, and pilot-controlled braking are typically controlled by separate functions. When separated, it is possible for one type of calculated deceleration function, such as antiskid, to interfere with the calculation of another deceleration function, such as the autobrake function. Among the many challenges that must be overcome in designing an antiskid braking system are: the set point for an antiskid brake control is unknown because actual surface coefficients of friction are unknown; the system is unidirectional in that the wheel can only be slowed down by the brake; and system must have a brake with a response lag small enough to manage wheel locking that might occur after 2.5-5 milliseconds (ms) of braking.
In addition, determining the appropriate amount of skidding and the appropriate reference velocity can be particularly problematic. The appropriate amount of skidding is described by the much discussed but seldom measured mu-slip curve. Typically such curve is represented by the coefficient of friction μ (mu) between the wheel and the running surface on a vertical axis and the slip ratio on the horizontal axis. A slip ratio of zero is when the wheel is not skidding while a slip ratio equal to one represents a fully locked wheel.
The amplitude and peak location of the mu-slip curve unfortunately can vary substantially for different running surfaces or even the same running surface. A lower amplitude mu-slip curve may represent an ice or water patch. Ideally, the antiskid brake control system should allow the wheel to slip at the peak of the mu-slip curve which provides the maximum stopping power. Antiskid brake control systems are commonly accepted to be ninety percent efficient which means that, on average, the control system should be within ten percent of the mu-slip peak regardless of the value or location of the peak. However, since the mu-slip curve depends on so many variables (e.g., and without being limited thereby, tire tread groove pattern, tire tread compound, temperature, tire pressure, running surface material and finish, etc.), the mu-slip curve begins to resemble a random variable. This makes it difficult for conventional antiskid brake control systems to track adequately the peak of the mu-slip curve.
Recently there have been efforts to utilize optimal state estimation techniques, such as Kalman filters, in antiskid brake control systems. For example, U.S. Pat. No. 4,679,866 to van Zanten et al. discusses a method for ascertaining a set-point braking moment using a Kalman filter. U.S. Pat. No. 4,715,662 to van Zanten et al. describes a method for determining an optimal slip value using a Kalman filter. In addition, U.S. Pat. No. 6,220,676 to Rudd describes a sophisticated system and method for antiskid control and is incorporated herein by reference.
Autobrake systems typically apply the same pressure to each brake. Such application can create a lack of yaw stability because brake friction can vary substantially from brake to brake. The variation in brake friction can be due to differences in material, wear or temperature. Assume for the moment that brakes on the left side of the vehicle have twice the friction as those on the right. Equal pressure applied to each brake would result in twice as much torque and drag on one side of the vehicle than on the other. The unbalanced drag would cause the vehicle to veer sharply toward the side with increased torque. In an airplane, even if a pilot were able to keep the aircraft on the runway using the nose wheel, the uneven brake torque would cause the left wheel to heat up much more than the right. A hot brake takes longer to cool, thereby causing a potential delay in the departure of the plane's next scheduled flight.
Antiskid operation can also cause a lack of yaw stability, depending on the type of hydraulic valve used. If a cross wind occurs at high speed where nose wheel steering is disabled, a pilot would compensate for the wind by altering brake pressure. For example, if the cross wind causes a plane to veer left, the pilot can release pressure from the left brake pedal so that more brake pressure exists on the wheel than on the left, causing the plane to veer right and compensate for the cross wind. However, the pilot may be asking for three thousand pounds per square inch (psi) but the antiskid has determined that the runway will only support two thousand psi of hydraulic pressure. The pilot has to reduce pressure one thousand psi on the inside brake before any directional control occurs and during this time stopping distance is increasing.
Another situation where differential braking is desirable is when the left side of the vehicle is on a low friction surface and the right on a high friction surface where the pilot is requesting full pressure. The vehicle will normally veer to the right. The pilot will have to reduce the right pedal to regain directional control. Unfortunately, the right brake is capable of providing most stopping power in this situation so that stopping distance must necessarily increase at a time when the pilot has decided that minimum stopping distance is desired.
In view of the aforementioned shortcomings associated with conventional antiskid brake control systems, there is a need in the art for a combined brake control, autobrake and antiskid control system capable of accurately and reliably ascertaining and implementing relevant parameters.